The present invention relates to a DC-DC converter of the chopper type, and more particularly to a DC-DC converter provided with soft starting function suitable for minimizing its size and reducing its manufacturing cost. The DC-DC converter of this type is suitable to be used for an electric power source or the like of portable devices adapted to be operated by a battery, and further it is suitable to drive an electromagnetic relay, a compact electric motor, an EL (Electro-Luminescence) or the like through a battery.
Hitherto, there are known a DC-DC converter provided with a soft starting circuit for the purpose of a restriction or the like for a rush current (JP-A-5-76167), and a DC-DC converter provided with not only a soft starting circuit but also an over current detection circuit (JP-A-7-298614). A PWM (Pulse Width Modulation) system for varying the width of a pulse for controlling a switching operation and a PFM (Pulse Frequency Modulation) system for varying the switching frequency are a precondition for those DC-DC converters.
Moreover, there is known a reference voltage generating circuit suitable to be easily structured by IC elements, which utilizes the band gap voltage power source with a precise voltage output but an insufficient output current. Such reference voltage generating circuit 5, as shown in FIG. 4, is structured so that the reference voltage Vref generated by a band gap voltage power source 5a is amplified for current by a voltage follower 5b or the like in order to supply it to necessary circuits.
In contrast, since there is a boundary of minimizing the size and decreasing the manufacturing cost for the conventional DC-DC converters in which PWM and PFM are made precondition because of complexities of its circuit structure and its adjusting operation, there is presented by the same applicant as the present application, Japanese Application No. 10-322720 of the type (referred to as an oscillation stopping/restarting type hereinafter) in which the oscillation of an oscillation circuit for generating a fixed frequency is stopped suitably. According to the type, it is only sufficient to adapt an operation property of the circuit to a switching condition utilizing a predetermined frequency thereby enabling PWM and VCO unnecessary.
Namely, as shown in FIG. 3, for the purpose of boosting or decreasing an input voltage Vin from a DC primary electric power source 1 to a desired output voltage Vo to be supplied to a load 4 connected to a smoothing circuit 3, the DC-DC converter 2 of the oscillation stopping/restating type is composed of an oscillation circuit 2c for oscillating a predetermined frequency and generating a pulse signal A with a predetermined pulse width, a switching circuit 2b for chopping the inertia current I flowing into an inductance element section 2a on the basis of the input voltage Vin from the DC primary electric power source 1, in accordance with the pulse signal A, and a feedback control circuit 2d for generating a feedback control signal B on the basis of the output voltage Vo obtained by smoothing the output from the switching circuit 2b, and feeding it to the oscillation circuit 2c thereby stopping/restarting the oscillation.
However, in the case where it is tried to provide a soft starting function for the DC-DC converter having the above-mentioned structure, it is difficult to adopt the conventional measure as it were and further it is not easy to provide the soft starting function therefor, because a switching control type as a premise is different form each other,
Therefore, it is a technical problem as to how the function for preventing and controlling the rush current and the over current is provided for the DC-DC converter of the oscillation stopping/restarting type.